Allied Air 4HP16LT Series Installation And Maintenance Instructions Manual page 17

OPERATION
Outdoor unit and indoor blower cycle on demand from the
room thermostat. When the thermostat blower switch is
moved to the ON position, the indoor blower operates
continuously.
Filter Drier
The unit is equipped with a large capacity bi-flow filter
which keeps the system clean and dry. If replacement is
necessary, replace with one of similar design and
capacity. The replacement filter drier must be suitable for
use with R410A refrigerant.
Crankcase Heater
If unit is equipped with a crankcase heater, it should
be energized 24 hours before unit start-up to prevent
compressor damage as a result of slugging.
Emergency Heat Function (Room Thermostat)
An emergency heat function is designed into some room
thermostats. This feature is applicable when isolation of
outdoor unit is required or when auxiliary electric heat is
stage by outdoor thermostats. When the room thermostat
is placed in the emergency heat position, the outdoor unit
control circuit is isolated from power and the field-supplied
relays bypass the outdoor thermostats. An amber
indicating light simultaneously comes on to remind the
homeowner that the unit is operating in the emergency
heat mode.
Emergency heat is usually used during an outdoor
shutdown, but it should also be used following a power
outage if power has been off for over an hour and the
outdoor temperature is below 50°F. System should be left
in the emergency heat mode at least 6 hours to allow the
crankcase heater sufficient time to prevent compressor
slugging.
DEMAND DEFROST SYSTEM DESCRIPTION
The demand defrost controller measures differential
temperatures to detect when the system is performing
poorly because of ice build-up on the outdoor coil. The
controller "self-calibrates" when the defrost system starts
and after each system defrost cycle. The defrost control
board components are shown below.
The control monitors ambient temperature, outdoor
coil temperature, and total run time to determine when a
defrost cycle is required. The coil temperature probe is
designed with a spring clip to allow mounting to the
outside coil tubing. The location of the coil sensor is
important for proper defrost operation.
506468-01
NOTE - The demand defrost board accurately
measures the performance of the system as frost
accumulates on the outdoor coil. This typically will
translate into longer running time between defrost
cycles as more frost accumulates on the outdoor coil
before the board initiates defrost cycles.
DIAGNOSTIC LEDS
The state (Off, On, Flashing) of two LEDs on the defrost
board (DS1 [Red] and DS2 [Green]) indicate diagnostics
conditions that are described in table.
Mode
No power to control
Normal operation/
power to control
Anti-short cycle
lockout
Low pressure switch
fault (Optional)
Low pressure switch
lockout (Optional)
High presssure switch
fault (Optional)
High pressure switch
lockout (Optional)
Issue 1004
Green Led (DS2)
Red Led (DS1)
OFF OFF
Simultaneous Slow Flash
Alternating Slow Flash
Slow FLASH
OFF
ON
OFF
Slow FLASH
OFF
OFF OFF
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